The present invention relates, in general, to temperature-sensing apparatus and technique for thermic reactors and, in particular, to a new and useful arrangement and technique for determining the existence of hot or cold spots in a chemical reactor.
The most economical operating temperature for an isothermal reactor is one at which the highest conversion of the primary feed stream occurs. For an exothermic (endothermic) reaction, catalyst selectivity increases (decreases) as the reaction temperature is lowered, and conversion increases (decreases) as reaction temperature is increased (decreased). Thus, temperature requirements for high selectivity and high conversion are opposed to each other. Consequently, a narrow temperature band is available for economic operation of a reactor. Furthermore, catalyst selectivity is not necessarily uniform throughout a reactor due to catalyst manufacturing and quality control techniques employed therein. This, coupled with flow characteristics, catalyst packing, reaction type and operating temperature range may result in the establishment of hot (cold) spots in a reactor.
The occurrence of hot (or cold) spots in a reactor may lead to any or all of the following problems:
1. Catalyst poisoning due to high temperature. This may result in either reducing catalyst life, or the requirement for frequent catalyst regeneration or new catalyst depending on the specific condition. PA1 1. Since only longitudinal temperature profile is measured, determination of maximum and minimum temperature does not necessarily reflect the true maximum and minimum temperatures in a reactor.
2. Reduction in conversion to desired products. Consequently, load on down stream purification units is increased.
3. Cooling (heating) fluid demand increases to meet the requirement of additional heat removal.
4. Occurrence of unsafe conditions depending on specific reactions and products.
5. Requirement for increased operator attention for unit control.
Hence, it is necessary that hot and/or cold spots in a reactor be identified quickly and accurately, and the information used in the control system for the reactor.
It is known to detect hot or cold spots in thermic reactors by using multiple temperature sensors that are installed across the length of the reactor. This gives a longitudinal temperature profile across the reactor axis. Also, maximum and minimum temperatures are determined for further use in the control system.
The drawbacks of the state of the art methodology, as described above, are:
2. As true hot and cold spots may lie, beyond the temperature sensor locations, on a radial axis, the state of the art method does not provide an accurate means of detecting hot and cold spots.
3. In view of reactor dynamics and significantly large delay time of temperature sensors, significant time elapses before the temperature around the measuring point reaches the hot or cold region temperature. By this time, the temperature of the hot or cold spots has further increased or decreased due to the temperature profile. Hence, temperature may further increase by the time detection is made, thus placing the reactor operation in an undesirable regime.
Pertinent references which disclose temperature sensing techniques are found in U.S. Pat. Nos. 3,061,415 to Braconier et al; 3,079,372 to Falknier et al; 3,830,698 to Kleiss; and 4,008,049 to Clemmer et al.
The Falknier patent relates to a process control system wherein temperatures throughout the length of the reactor are displayed on a cathode ray oscilloscope. This reference disclosed the utilization of a plurality of thermocouples at desired intervals along the length of the reactor tubing.
The Kleiss et al reference relates to a method and apparatus for controlling the temperature in a fractionation column. This reference discloses the use of plural temperature sensors along the column length to provide meaningful measurements of the temperature gradient within the column.
The Braconier et al reference relates to a process for monitoring the progress of gas phase reactions by the use of temperature measurements in key positions and the Clemmer et al reference relates to the control of processes by detecting and measuring temperatures at key points in the process.